Unless you have high cholesterol or triglycerides, taking niacin will do you more harm than good. It will have no effect on your ability to pass a drug test, any drug test. The flushing you are hoping is like flushing a toilet, removal of waste. The flushing caused by niacin is similar to blushing, the dilation of capillaries causing your skin to turn red. Two different meanings for the same word.

No. It doesn’t do a damned thing. This is one of those substances that for some reason people have been utterly fixated on despite its more or less complete lack of utility for what they want. Niacin and apple cider vinegar have become sort